1. Field of the Invention
This invention relates to the fabrication of membrane-based cassettes and stacks made via encapsulation of the component parts by a resin or thermoplastic matrix. The present invention includes an internal porting (e.g., manifolding) feature which eliminates the need for a separate step to seal individual components prior to the assembly of the cell cassette. Cassettes and stacks of the invention are particularly well-suited for use in various electrochemical applications, including fuel cells, as well as ion-exchange applications.
2. Background
Electrochemical cells, and particularly, PEM fuel cells are well known in the art. PEM fuel cells convert chemical energy to electrical power with virtually no environmental emissions and differ from a battery in that energy is not stored, but derived from supplied fuel. Therefore, a fuel cell is not tied to a charge/discharge cycle and can maintain a specific power output as long as fuel is continuously supplied. The large investments into fuel cell research and commercialization indicate the technology has considerable potential in the marketplace. However, the high cost of fuel cells when compared to conventional power generation technology has deterred their potentially widespread use. Costs of fabricating and assembling fuel cells can be significant, due to the materials and labor involved, and as much as 85% of a fuel cell's cost can be attributed to manufacturing.
A single cell PEM fuel cell consists of an anode and a cathode compartment separated by a thin, ionically conducting membrane. This catalyzed membrane, with or without gas diffusion layers, is often referred to as a membrane electrode assembly (“MEA”). Energy conversion begins when the reactants, reductants and oxidants, are supplied to the anode and cathode compartments, respectively, of the PEM fuel cell. Oxidants include pure oxygen, oxygen containing gases, such as air, and halogens, such as chlorine. Reductants, also referred to herein as fuel, include hydrogen, natural gas, methane, ethane, propane, butane, formaldehyde, methanol, ethanol, alcohol blends and other hydrogen rich organics. At the anode, the reductant is oxidized to produce protons, which migrate across the membrane to the cathode. At the cathode, the protons react with the oxidant. The overall electrochemical redox (reduction/oxidation) reaction is spontaneous, and energy is released. Throughout this reaction, the PEM serves to prevent the reductant and oxidant from mixing and to allow ionic transport to occur.
Current state of the art fuel cell designs comprise more than a single cell, and in fact, generally combine several MEAs, flow fields and separator plates in a series to form a fuel cell “stack”; thereby providing higher voltages and the significant power outputs needed for most applications. Depending on stack configuration, one or more separator plates may be utilized (referred to as a “bipolar stack”) as part of the stack design. Their basic design function is to prevent mixing of the fuel, oxidant and cooling input streams within the fuel cell stack, while also providing stack structural support. These separator plates serve as current collectors for the electrodes and may also contain an array of lands and grooves formed in the surface of the plate contacting the MEA, in which case the separator plates are often referred to only as “bipolar plates” and the array of lands and grooves as “flow fields”. Alternatively, the flow field may be a separate porous electrode layer. Ideal separator plates for use in fuel cell stacks are thin, lightweight, durable, highly conductive, corrosion resistant structures that can also, if desired, provide effective flow fields and thereby become bipolar plates.
In the flow fields, the lands conduct current from the electrodes, while the grooves between the lands serve to distribute the reactants utilized by a fuel cell, such as hydrogen, oxygen or air, evenly over the faces of the electrodes. The channels formed by the lands and grooves also facilitate removal of liquid reaction byproducts, such as water. A thin sheet of porous paper, cloth or felt, usually made from graphite or carbon, maybe positioned between each of the flow fields and the catalyzed faces of the MEA to support the MEA where it confronts grooves in the flow field to conduct current to the adjacent lands, and to aid in distributing reactants to the MEA. This thin sheet is normally termed a gas diffusion layer (“GDL”), and can be incorporated as part of the MEA.
Fuel cell stacks may also contain humidification channels within one or more of the coolant flow fields. These humidification channels provide a mechanism to humidify fuel and oxidants at a temperature as close as possible to the operating temperature of the fuel cell. This helps to prevent dehydration of the PEM as a high temperature differential between the gases entering the fuel cell and the temperature of the PEM causes water vapor to be transferred from the PEM to the fuel and oxidant streams. The location of the humidification channels can either be upstream from the MEA, such as in the fuel cell stacks described in U.S. Pat. No. 5,382,478 to Chow et al., and U.S. Pat. No. 6,066,408 to Vitale et al., or downstream from the MEA, such as those described in U.S. Pat. No. 5,176,966 to Epp et al.
Of necessity, certain stack components, such as the GDL portion of the MEA, are porous in order to provide for the distribution of reactants and byproducts into, out of, and within the fuel cell stack. Due to the number of elements within the stack, a means to prevent leakage of any liquid or gases between stack components (or outside of the stack) is needed. To this end, gaskets or other seals are usually provided between the surfaces of the membrane and/or MEA and other stack components, such as flow fields, and on portions of the stack periphery. These sealing means, whether elastomeric or adhesive materials, are generally placed upon, fitted, formed or directly applied to the particular surfaces being sealed. These processes are labor intensive and not conducive to high volume manufacturing and add to the high cost of fuel cells. The variability of these processes also results in poor manufacturing yield and device reliability.
Fuel cell stacks range in design depending upon power output, cooling, and other technical requirements, but may utilize a multitude of MEAs, seals, flow fields, and separator plates, in intricate assemblies that result in manufacturing difficulties and further increased fuel cell costs. For example, one fuel cell stack, described in U.S. Pat. No. 5,683,828, to Spear et al., employs bipolar plates containing up to ten separate layers adhesively bonded together, each layer having distinct channels that are dedicated to passing cooling water through the fuel cell stack for thermal management.
These multitudes of individual components are typically assembled into one sole complex unit to form the fuel cell stack. The stack is compressed, generally through the use of end plates and bolts, although banding or other methods may be used, such that the stack components are held tightly together to maintain electrical contact there between. These current means of applying compression add even more components and complexity to the stack and pose additional sealing requirements. Various attempts have been made in the fuel cell art to cure these deficiencies in fuel cell stack assembly design and thereby lower manufacturing costs.
Other conventional processes are described in U.S. Pat. No. 6,080,503, to Schmid et al., U.S. Pat. No. 4,397,917, to Chi et al., U.S. Pat. No. 5,176,966, to Epp et al. However, notable disadvantages have been associated with such conventional processes.
See, e.g., U.S. Pat. No. 6,080,503, to Schmid et al., which describes the replacement of gasket based seals within certain portions of the stack with an adhesive based material in the form of tapes, caulks or layers. However, assembly of that stack still requires manual alignment of the components during the adhesion process, in a manner not unlike caulking a seal, and sealing only occurs at those interfaces where adhesive has been applied through active placement.
Similarly, U.S. Pat. No. 4,397,917, to Chi et al., describes the fabrication of subunits within a fuel cell stack and is reported to provide ease in handling and testing. However, this design relies on conventional sealing among the components and between subunits. In addition, no manifolds internally penetrate the subunit.
See also, U.S. Pat. No. 5,176,966, to Epp et al., for its method of forming at least some of the required gaskets directly into the fuel cell stack assembly; U.S. Pat. No. 5,264,299, to Krasij et al., describes a fuel cell module having a PEM interposed between the two porous support layers which distribute reactant to the catalyst layers in which the peripheral portion of the support layers are sealed with an elastomeric material such that the PEM is joined with the support layers and the open pores of the support layers are filled with the elastomeric material making it fluid impermeable.
In contrast to these and other conventional processes, the present invention builds on the fuel cell cassette and method of manufacturing a fuel cell cassette described in our World Publication WO 02/43173 based on U.S. patent application Ser. No. 09/908,359 entitled, Electrochemical Polymer Electrolyte Membrane Cell Stacks and Manufacturing Methods Thereof, which application is incorporated herein by reference.
Briefly, WO 02/43173 detailed a three step process for the formation of fuel cell cassettes which included the following:                1) Sealing of unused manifold openings/ports on each of the particular flow fields (fuel, oxidant, and coolant). For example, in the case of the oxidant flow field, ports utilized for the distribution of fuel and coolant (on other layers) must be sealed about their perimeter to prevent the mixing of these input streams.        2) Sealing of all the ports within the membrane electrode assemblies (MEA) to prevent the leakage of the reactants within the MEA layers.        3) Layering these components (appropriately sealed as described) within a mold or fixture in a method prescribed by the particular stack design. Once the pieces are assembled within the fixture, a resin is introduced about the periphery. Using vacuum transfer molding or injection molding techniques, the resin is forced into the edges of the cassette assembly. Once hardened, it provides structural support and edge sealing over the assembly.        
The resulting fuel cell cassette is then transformed into a fuel cell stack with the addition of end plates. Such a construction provides appropriate manifolding and a means of compression.
However, despite the numerous conventional processes available to those skilled in the art, and even in light of our own advancements in this field, there remains a need for improved cassettes and stacks. In particular, it would be highly desirable to develop fuel cell stacks and cassettes with enhanced reliability, and with further reductions in labor and costs.